Absorbent articles such as sanitary napkins, panty liners, catamenials, incontinence inserts and diapers for adults or babies are commonly provided with an adhesive on their garment-facing surface to attach them during their usage period to a garment of the user, for example a pressure sensitive, hot melt adhesive. These adhesives are typically covered with a release paper prior to use. The adhesives thus need to stick to the garments well, to ensure the articles do not move during use.
More generally absorbent articles are provided with adhesive areas in order to combine the components, which ultimately make up part or the whole of the absorbent article. In particular, multi-layer structures forming the topsheet, core or backsheet are often combined by adhesives called construction adhesives.
Typically these products are made by high-speed machinery. Current machinery includes equipment, such as spray guns or slot coaters, which continuously or intermittently add the adhesive on the surface of an absorbent article. This needs to be done and can be done in a very fast manner, to ensure production at a very high speed. These types of processes are quite inflexible and inaccurate relative to the shape of the adhesive to be applied and they essentially can only provide the adhesive continuously. Alternatively, on/off systems for coating or spraying not only have the drawback of delay in their systems response but also generally the problems associated with accelerating and decelerating mass streams.
As alternative, more accurate printing processes using a printing rolls have been developed, examples of which are described in WO 96/38113 and EP 745 433-A; these documents describe to use a printing roll which rotates through an adhesive bath and contacts the surface of an absorbent article, which passes on the top of the coating roll. The roll can contain a specific pattern of cavities and is contacted with a scraper blade which scrapes off excess adhesive, so that mainly the cavities remain filled with the adhesive and thus, the pattern of the cavities can be transferred to the absorbent article.
As alternative process, WO 00/07533 suggest to replace the adhesive bath with a spraying tool or slot cater, positioned at the right or left hand side of the roll, which continuously applies an amount of adhesive onto a gravure printing roll with cavities, such that the cavities are filled to a certain extent only, which is then pressed against an absorbent article above the roll. A scraper blade is also provided which scrapes off any excess adhesive.
Even in these alternative method of printing an adhesive with a roll, such as gravure printing, it has been found that it is difficult to apply sticky, stringy adhesives with precision, such that for example the adhesive is about completely transferred from the roll, the exact required amount of adhesive is applied with precision, or for example all cavities, present on the printing roll, contain the exact required amount of adhesive and this is about all transferred, e.g. if the volume of all the cavities is the same, such that each cavity contains an equal amount of adhesive. Another major problem with the printing processes of the prior art is that the used techniques frequently result in stringing of the adhesive during application onto the article; the resulting strings are a form of contamination on the articles, typically resulting in an uneven pattern of the adhesive. This is in particularly the case when the process is performed at a high speed, such as normally necessary in economically feasible production processes, e.g. of more than 20 m/min, or even more than 100 m/min or even more than 200 m/min.
Furthermore, these known methods are such that the adhesive typically has to be heated to very high temperatures, in order to be sprayed, and that the temperature of the adhesive and the roll (or the difference between these temperatures) is difficult to control. Also, whilst slot coaters can apply the adhesive very finely, the applied adhesive tends to clump together, seeking to minimize surface area. Thus, an uneven application is obtained in practice. Furthermore, adhesive applied with slot coaters tends to fly off the rotating roll after application, especially when the adhesive clumps together and/or when they are very hot and more viscous.
Hence, there still exists a need to provide an improved (continuous) high speed process to apply adhesives or other sticky materials to articles, which addresses the problem of the known printing techniques, as discussed above, and thus, provides an even more accurate and efficient way to apply sticky materials, such as adhesives, onto articles, typically in a shaped designs.
The inventors found that this is achieved by using a specific transfer tool, preferably a printing roll, with specific surface features and/or adhesives with specific properties. In more detail, the inventors have found that the adhesive has to have certain properties to be sticky enough to ensure an article adheres to the garments of the user or to adhere two articles or two parts of articles together, whilst still transferable, in particular whilst not causing stringing during the transfer (printing) process and whilst being transferred about completely from the transfer tool onto the article. Whilst it has been recognized in the prior art that there is a potential conflict between the requirements for adhesives to be transferable or printable and also to be able to adhere sufficiently to the garment, neither any further understanding of this conflicting problem nor a solution has been found to date; the printing rolls and adhesives used for the (production of) absorbent articles described in the prior art do not have the properties of the presently used printing rolls and adhesives.
The process of the invention thus provides a much more accurate and efficient application of the sticky material onto the article. Furthermore, the process of the invention, using a transfer tool with a specific surfaces and/or a specific adhesive, results in a significantly reduced level of contamination by stringing or even no stringing at all, i.e. due to the nature of the transfer tool and/or the adhesive used herein, stringing can be (almost) avoided. This thus also helps to ensure the adhesive is applied exactly as required, e.g. as a completely evenly applied layer, or in a very specific pattern, for example exactly corresponding to the pattern of cavities on the transfer tool. The articles obtained by this process thus have a much more uniformly applied layer or (dot) pattern of the sticky material, such as the adhesive, compared to the absorbent articles of the prior art.